Chapter 10. Issues
Related to Wood

This chapter addresses a number of issues related to wood as a
material used in covered bridges. The discussion does not duplicate
information readily available in common references. Instead, it
attempts to clarify some topics that are of special interest to
covered bridge designers and contractors.

It is important to be definite when specifying wood to be used
in covered bridges.

"Wood" usually simply indicates material cut from trees. The
word can imply reference to any size structural component.

"Wooden" technically indicates objects or structures made of
wood. Hence, covered bridges are wooden structures. Yet, in common
usage, covered bridges are often also called timber or
timber-framed structures.

"Timber" is used in a number of ways when discussing covered
bridges. Timber often denotes larger-sized wooden members used in
structures. The NDS defines timbers as those components at least
127 mm (5 inches) thick. Timber also is used to describe standing
trees before harvest. "Timber Engineering" usually refers to
structural engineering that specializes in wood products; those
working with buildings may refer to it as wood engineering.

"Lumber" is used in confusing ways. Technically, lumber can
indicate any size structural component, usually solid-sawn, as
compared to manufactured with wooden subassemblies. Lumber commonly
refers to wood products with sizes smaller than timbers. The NDS
cites dimension lumber as that with thickness from 50 to 100 mm (2
to 4 inches) to differentiate it from the larger timbers.

A common question that often arises when discussing extant
covered bridges is, "What species of wood was typically used in
these bridges' construction?" Although Douglas Fir and Southern
Pine are the most popular species for more modern structural
applications, most historic covered bridges were constructed with
softwoods found fairly close to the bridge site. Eastern Hemlock,
White Pine, and Spruce are commonly found in those bridges in the
East. Douglas Fir was used in almost all western bridges. The
southern covered bridges were built mostly with Southern Yellow
Pine.

As important as the species is to the evaluation of covered
bridges, the quality of the wood used must be put in context.
Extant covered bridges, especially in the East, were often built
with timbers fashioned from logs cut in the magnificent
first-growth eastern forests. The timber cut in the 18th
and 19th centuries differs significantly from the timber
on which modern timber design codes are based. Note that while we
say the timber is different, the wood itself generally has not
changed much, if at all. A Colonial-era Eastern Red Spruce is
genetically indistinguishable from today's pulp logs. What have
changed enormously are the forests and the trees from which those
forests are made. Those first-growth, natural primeval forests were
full of relatively wide-spaced, immensely tall, very thick trees.
Their branches, or canopies, were far above the ground, the result
of centuries of competition with their neighbors for sunlight. The
competition was tough enough that the trees grew slowly, once they
were of any size. The resulting wood had very tightly spaced annual
rings, or tight grain. Because the canopies were so high, the logs
were long, with only a few of the branches that cause knots. Much
of these trees' trunks had the opportunity to evolve from sapwood
to the harder and more durable heartwood. In short, original
covered bridge builders had local access to some of the best
structural timber in history.

Another issue important to the wood of extant historic covered
bridges is the fact that original timber often was cut from such
large logs that many of the members are free-of-heart-center
(FOHC), or did not need to include the heart of the tree within the
sawn member. Not only are those FOHC pieces free of the tree
center, or pith, which is weaker wood because it is the faster
growing juvenile wood, but also they tend to be far more stable in
use. The far more common boxed heart wood, which is found in
contemporary timber and is cut from smaller logs, is much more
prone to distortion and splitting as it seasons. This increased
movement in boxed heart timbers is caused by the differential
shrinkage rates between the radial and circumferential
directions.

The first-growth logs were often so straight and clear of
branches that the lumber cut from them generally has far fewer
imperfections, or grading defects, than found in the modern timber
assumed in current design codes. The most significant lumber flaw
to which these original trees were prone is the sloping grain that
comes from poor sawing practices or spiral grain in the trees. This
serious structural flaw can be found in the large timbers of
queenpost and kingpost trusses, but only rarely in the planks used
to frame Town lattice trusses. It might be assumed that those
planks dried quickly enough to show their flawed grain as sloping
checks in time to be replaced before truss fabrication, and that
they were cheap enough that the builders were willing to replace
the flawed planks and cut them up for less critical uses. In any
event, the covered bridges that have survived often show signs that
the builders were careful to have sorted the available stock for
high-quality timbers.

Chapter 13 discusses the issues involved in establishing
allowable timber member stresses when evaluating covered bridge
capacities. That section includes more material about dealing with
the extant old-growth timber versus modern timber components.

The availability of wood components for repair or rehabilitation
is sometimes neglected in the early plan development phase of
covered bridge work. The extant bridge often was built with timber
components of lengths and cross sections that are larger than what
is currently available. For instance, a recent reconstruction
project in Upstate New York (the Hamden Covered Bridge in Delaware
County, NY) involved replacing the bottom chords of a Long truss.
The original Eastern Hemlock members were 230 by 330 mm (9 by 13
inches) wide by about 15.2 m (50 feet) long. They would have been
cut from an impressive tree, even for the time of the construction
in 1856.

A bridge in central Vermont (the Mill Bridge in Tunbridge, VT)
unfortunately was destroyed by ice floes in March 1999. The
original challenge was to replace it generally in-kind with local
softwoods to pay homage to the original construction. However,
modern design requirements and loads, coupled with current
allowable stresses, meant that local hemlock was not available in
the sizes and grades to satisfy the design criteria. Ultimately,
the bridge was rebuilt with Douglas Fir imported from the western
United States.

Part of this problem can be traced to the tendency of the NDS to
list and describe timber materials and sizes that may not actually
be available in various regions (e.g., Select Structural Eastern
Hemlock or Dense Structural 86 Southern Pine). A designer might
find timber species and grading information in the NDS and proceed
with the design accordingly. Then, when challenged to help the
contractor find the specified material, the engineer finds that it
is not available after all, or it is cost-prohibitive.

Therefore, it is important to consider timber availability
before beginning the design process. A minimum appropriate effort
might be to contact potential timber providers early in the
project. There are only a relative few in the United States that
provide the bulk of larger (and higher allowable stress) timber
components. The designer might also discuss potential project needs
with contractors experienced with covered bridge work. Contact
between designer and contractor requires consideration of potential
conflicts during the bidding process, but the improvements in the
bid documents can be significant. Hence, one might solicit advice
from a contractor in another geographic area who does not intend to
bid on the project in question.

Another related issue involves choosing a timber species for any
replacement components. In many instances, new components are
chosen based almost solely on strength properties. Fresh-sawn
Southern Pine and Douglas Fir have similar strength properties, yet
Douglas Fir is more available in larger sizes and lengths. Some
timber craftsmen prefer Douglas Fir to Southern Pine as a more
workable wood. Others, however, find that fir can be more prone to
splitting than pine. Again, it is best to check early in the
process with those who specialize in this type of work (or, at
least, with this type of material) to determine the preferred
timber replacement material in the geographic area of the
project.

Existing covered bridges often were built with green wood and
usually have served well, despite the lack of intentionally dried
wood. Those dealing with the rehabilitation of a covered bridge
must decide whether to use dried or green wood. This section
presents both sides of a controversial issue.

Green wood can mask inherent weaknesses in timber, such as
inclined grain.

Green wood will shrink after installation, while seasoned
timber does not change size in any appreciable way (if sheltered
from the direct weather).

Green wood is slightly more susceptible to degradation from
fungi and insects.

Green wood does not retain wood preservatives and paint/stain
as well as dried wood.

The Wood Handbook: Wood as an Engineering Material,
published by the United States Forest Service, offers an excellent
discussion on drying wood.[10]

For all these reasons, it is desirable to use seasoned wood in
covered bridge work, even if seasoning takes time. Thinner
materials (up to 100 mm (4 inches) thick) can be kiln dried, a
process that can take up to a week or more. It has been impractical
to attempt kiln drying of larger timbers; hence, it is common
practice to either use green material in projects (which can result
in unplanned stresses during in-place drying), or to specify
seasoned material.

Radio frequency, or microwave, kilns are now available for
drying large timbers in relatively short times, albeit at
considerable expense. Previously thought to cause little undue
damage by rapid drying, there are recent reports of some problems
with internal honeycombing by microwave kiln drying. These new
kilns are found mostly in the Pacific Northwest. Even with careful
and sophisticated restraints during drying, many timbers may
require a second sawing before being suitable for fabrication.
Shrinkage in the kiln, subsequent resurfacing, and some timber loss
in the drying process all mean that the original timber order will
need to be bolstered to compensate for these losses.

Dried timber is better to use in repairing existing structures.
However, it is important to consider the cost of using dried
timbers versus the consequences of using green timber. On a timber
order of the size common for a major rehabilitation project of a
covered bridge, the cost of kiln-dried timber will be much more
than green. In some cases, it could be double the cost of that for
unseasoned, fresh-sawn, timbers.

Another way to obtain large timbers that are already dry is to
use recycled, or salvage, timbers. Many industrial structures were
framed with heavy Douglas Fir or Southern Yellow Pine. These
buildings are being dismantled to make way for other projects,
mostly in urban areas. Rather than simply hauling the material to
landfills, many demolition contractors are finding that it pays to
dismantle the structures and sell the timbers to reprocessors.
These timbers are generally high-quality and cut from first-growth
logs. They also usually have bolt holes and notches that bear
consideration. The industry is slowly developing standards and
grading rules for using and establishing allowable stresses in
these recycled timbers. The economics involved with specifying
recycled timbers are unpredictable, at best. A lot of recycled
timber may be available at any time, but this timber will rarely
have the dimensions that the designer needs. The raw material price
can also seem low, but hauling, stripping hardware, resawing, and
sorting can add much to the in-place costs. The material cost of a
recycled timber, in place, might be higher than for glulam timbers,
for example.

To demonstrate the design penalties involved with using nondried
wood, consider the NDS specifications for sawn materials (refer to
the commentary in section 2.3.3 of the NDS, "Wet Service Factor").
Those components subject to moisture contents higher than 19
percent during service are subject to capacity reductions unique to
the stress type being investigated. The largest adjustment factor
of 0.67 is for compression perpendicular to grain, meaning that
only 67 percent of the basic value can be used. Although
compression perpendicular to grain is not encountered commonly
during routine design, adjustment factors of 0.80 for compression
parallel to grain and 0.85 for bending are specified; those types
of stresses are commonly encountered in covered bridge design.

NDS specification reduction factors for glulam components are
even more restrictive. For moisture contents in excess of 16
percent during service, the allowable stress adjustment factors are
0.53 for compression perpendicular to grain, 0.73 for compression
parallel to grain, and 0.80 for bending.

Covered bridges rarely contain elements subject to moisture
conditions during service that approach the upper limits noted
above; therefore, this issue is almost moot. However, in some rare
instances, the lower elements (bottom chords and floor beams) of
bridges with very low clearance above water may be exposed to
higher moisture contents during service than desired. Accordingly,
it is prudent in those instances to consider the moisture penalties
of the NDS specifications.

While not directly related only to material issues, the use of
green wood in rehabilitating a covered bridge should include
provisions that require the rebuilding contractor to return to the
structure to retighten all bolts and wedges, say, after 6 months
shrinking and settling. The period would depend on the as-installed
moisture contents. Further, the project schedule should provide
adequate time for drying, should it be required.

Terminology related to cross-sectional sizes of members deserves
clarification. Historic covered bridges were built before industry
standards that addressed sizing and finishing. Some terms commonly
used include:

Nominal size is the most common type of cross-sectional
identification. Nominal sizing refers to the rough-sawn, initial
cut size of the timber, before any drying or finishing.

Dressed (or finished) timber has been further processed to
plane the sides; this provides a finished, or smoother, appearance
and helps "true-up" the timber to parallel sides (the initial rough
saw cut may have resulted in slight thickness variations along the
length of the member). This finishing typically removes up to 12 mm
(0.5 inch) of thickness.

Full-sawn timbers are actually the same size as their nominal
size would suggest. Hence, full-sawn timber is cut from larger
dried timber so that the full-size timber will not shrink further
after drying. Full-sawn timbers are either used rough, directly off
the sawmill, or are custom-sawn oversized so that they are still
full size after planing. The rough, off-the-sawmill timbers are
cheaper than planed nominal timbers.

Although covered bridges use heavier timbers, the lighter
dimensional lumber framing industry provides insight in this
regard. It is commonly recognized that a "2 by 4'" (50 by100 mm) is
not actually 2 inches by 4 inches. In fact, the real size of
finished 2 by 4s has changed over time. For a long time, they were
typically finished at 1 5/8 by 3 5/8 inches (41 by 92 mm). More
recently, the size has been reduced to 1 1/2 by 3 1/2 inches (38 by
89 mm). Hence, the 2 by 4 dimension is the nominal designation. The
1 1/2 by 3 1/2 inches dimension is the finished, or dressed,
size.

Similarly, for many years the finished size of heavy timbers (up
to 150 mm (6 inches) nominal) has been 12 mm (0.5 inches) smaller
than nominal. For larger members, the attendant drying and planing
losses have made for a 19-mm (0.75-inch) reduction from nominal to
actual. The NDS supplement includes a comprehensive table of the
standardized nominal sizes and their actual cross-sectional
dimensions.

The sizing convention is important on covered bridge projects,
because almost all the timbers used in a covered bridge project are
unfinished (with the possible exception of the siding installed
during subsequent rehabilitation). Further, because the focus of
this manual is on historic (existing) covered bridges, the work
discussed here deals with rehabilitation of existing bridges.
Therefore, the designer must deal with the measured in situ sizes
of existing members. Although new or added members may be different
sizes than the originals, the connections and details must be
compatible with existing conditions. As an example, if some chord
or lattice members in a Town lattice bridge are being replaced, the
new members should be the same thickness as the existing ones to
minimize the problems of the physical removal and replacement, and
in making connections.

A more common issue regarding timber sizing is the use of
full-size members versus nominal. In many cases, using full-size
members means a slight improvement in strength, which could be
important. Further, many timbers for covered bridge components are
resawn from larger components that have been previously harvested
and air-dried in larger cants. Therefore, it is often an advantage
to specify full-size members. For those larger members from fresh
but resawn timbers, the cost penalty for full-size members is small
or nonexistent.

Engineers should be careful when using precomputed geometric
properties of wood sections (e.g., A, Sx, and
Ix), because small changes in actual dimensions can lead
to large changes in those values.

Glulam components are fabricated from dimensional lumber that
has been dried and finished before gluing. The individual pieces
are often planed again, just before fabrication, to improve the
uniformity of their thickness and the glue-line quality. This means
that the depths of the fabricated member are well-defined, based on
the number of laminations of material used in its construction. The
current standard of the industry uses 35-mm- (1.37-inch) thick
laminations, and the component is made from Southern Pine
(reflecting that secondary planing step), while 38-mm- (1.5-inch)
thick laminations are used when the component is glued with
components from western species.

The width of glulam components is based on the size of the
individual pieces used in their construction. During the
fabrication process, the side faces of glued components can be very
uneven, due to slight unevenness in the widths and slight sweep and
twist in the individual pieces used. Excess glue is also squeezed
onto the side faces of the members. At the finished stage, standard
practice involves planing them after gluing to preestablished
widths (as specified by current standards) to remove the width
unevenness from the fabrication operation and to remove excess
glue.

Glulam components often are used to replace original solid-sawn
members in a covered bridge. The most common application for the
stronger glulam beams is for transverse floor beams. The glulam
member will rarely match the existing height dimensions and may
have to be cut to fit at the beam supports at the longitudinal
trusses. This material shift rarely represents any significant
challenge. However, if glulam components are used to replace truss
elements, then the actual widths of the members may become
important. Because the glulam members come in standardized and
established widths, one must carefully consider the ramifications
of these standard widths.

On a recent covered bridge rehabilitation project in Upstate New
York (the Hamden Covered Bridge in Delaware County, NY), the bottom
chords of a truss were to be replaced with glulam components. The
existing truss used pairs of vertical elements with three
horizontal chord elements that straddled the verticals at their
connections. The existing central chord element was rough-cut and
averaged 229 mm (9 inches) wide. The closest size of a standard
finished glulam element was 222 mm (8.75 inches) wide. It would
have been possible to replace a 229-mm- (9-inch) wide element with
a 222-mm- (8.75-inch) wide member, but it was also possible to
specify an industrial finish (in which the element is not planed
after gluing) for the glulam member. This effectively reduced the
amount of material that had to be removed in finishing the member
and meant that the final width was very close to the original 229
mm (9 inches).

Another excellent summary is contained in Timber
Construction for Architects and Builders.[11] Since
this manual focuses on aspects unique to covered bridges. the
following presents only an abbreviated discussion of this
topic.

As a basic introduction, wood preservatives are either oil-based
or water-based. Creosote, first used in the mid-1800s, was the most
commonly used wood preservative for a long time. However, the
difficulties of working with creosote within today's environmental
restrictions have made it less popular. Today the most popular
oilborne preservative for covered bridge use is pentachlorophenol,
or "penta." Some prefer waterborne preservatives; while chromated
copper arsenate (CCA) is one of today's most popular waterborne
preservatives, the arsenic component's effect on the environment is
a concern, and its use is becoming restricted. It is unclear at
this time which of the other waterborne preservatives will become
the preferred replacement treatment for CCA.

Although field application of wood preservatives is possible, it
is typically restricted to treating only field-cut surfaces.
Applied by brush, roller, or spray, the surface treatment is
relatively ineffective when compared to pressure impregnating the
preservative. The typical preservative-treated timbers used for
covered bridge materials are provided by specialty companies that
have invested in this special equipment. The pressure treatment
ideally is performed after initial cutting and drying, before
delivery to the project site. This means that all end grain
surfaces that are exposed during fabrication are also subject to
pressure treatment.

It is easy to recommend that all wood components used in
rehabilitation of covered bridges be treated with preservatives.
However, there are issues involving preservatives that make their
use somewhat controversial, or at least not automatic, for those
involved in specifying the work.

The original structural elements of most extant historic covered
bridges were built without preservative treatment. Some wood
species are more resistant to rot (e.g., White Oak, locust,
tamarack) and were used in critical locations such as floor
elements, bearing blocks beneath the chords at the abutments,
bottom chord elements adjacent to the abutments, and posts at the
ends of the bridge. Modern rehabilitation projects may consider use
of these species in lieu of pressure treatment.

The first widely used wood preservative, creosote, did not
become available until the mid-1800s. Preservatives would have
undoubtedly protected some covered bridges from early demise with
rot, but using preservatives is not nearly as important in
long-term performance as is maintaining effective siding and
roofing materials on the bridge, and covered bridge builders
understand this.

Rehabilitation projects often proceed with an initial order of
material based only on an investigation that was conducted with the
structure intact. After the rehabilitation is underway, it is
common to find additional deterioration in elements not originally
identified for replacement. Rarely do projects have enough time to
order additional materials at this midpoint and wait for the
pressure treatment process that is usually used for preservative
treatment. In some situations, the extra time for the pressure
treatment process may be unacceptable, and untreated wood is used
instead.

It is best to order extra components initially, to ensure that
ample materials are available when rehabilitation begins. With
luck, additional material does not need to be ordered in the middle
of the project. The use of preservatives is logical, in this
instance. When preservative-treated timbers are used, this is
especially helpful.

Another important design-related issue with wood preservatives
relates to one of the most popular species in this context, Douglas
Fir. The cell structure of fir is nonuniform in its composition,
thereby limiting fluid flow and making pressure treatment
inconsistent. The practice of incising the wood (cutting slits into
the surface) before pressure treatment greatly improves the
penetration of wood preservatives in firs, but this incising
structurally damages the elements; this structure is most important
to thin members. The 1997 NDS introduced a reduction factor of 15
percent for bending, tension, and compression stresses for incising
of dimension lumber. Dimension lumber is defined by NDS as material
from 50-100 mm (2-4 inches) thick. Hence, this reduction is not
relevant to heavy timber elements, but is of special interest for
the components of Town lattice trusses. This is a substantial
reduction and can easily lead to using Southern Pine as a
substitute, because it does not need to be incised before pressure
treatment.

Ideally, pressure treatment would be performed after all
fabrication cuts, due to the exposure-related health concerns of
cutting pressure-treated timber. However, cutting additional
pressure-treated members is required in many situations. In those
instances, appropriate safety precautions are required. Surface
preservative treatment applications are prudent after such
cuts.

There are occasional objections to the appearance of wood
treated with preservatives, as compared to the extant wood without
preservatives. The surface can, initially, have a greenish tint.
The incising of Douglas Fir produces a texture that does not look
particularly natural.

Closely related to preservative treatments in wood (discussed in
the previous subsection), some bridges are treated in place to deal
with specific infestations of wood-destroying insects. Bridges in
southern regions are prone to attack by termites or carpenter ants,
and routine treatment against this assault is appropriate. Many
bridges are subject to attack by powder post beetles, especially
the hardwood peg or trunnel components (see figure 79), in which
the surface holes are about 1.5 mm (0.06 inches) in diameter. Just
as other preservative treatments against fungi (or rot) are being
developed at an accelerating rate, chemical treatments for these
infestations are evolving rapidly. These in-place treatments are
usually applied with spray applicators.

Fire, whether by arson or from natural causes, has destroyed
many heavy timber structures, and covered bridges are not exempt.
Planners should make all reasonable precautions against fire loss
when investing in rebuilding a covered bridge. Various chemicals
have been used to treat wood products to reduce the rate of
material consumption during fire or to retard the start of fire
damage. The most effective applications use pressure treatment,
similar to the process for impregnating wood preservatives.
Unfortunately, many fire retardants introduced via pressure
processes reduce the strength of the treated member. Current design
specifications do not mandate specific reduction factors associated
with the use of fire retardant treatments; instead, they require
consultation with the manufacturer of the selected fire retardant
material. Further, for extant covered bridges, the fire retardant
must be applied in the field by spray or brush. At best, such
surface treatments are only marginally effective.

This has been a topic of intense activity for a number of years,
and an effective, field-applied treatment may be developed. When
this manual was published, FHWA was conducting research to identify
new generation fire retardant treatments for use on historic
covered bridges.

Protective finish treatments are used on the exterior siding of
many covered bridges (see figure 80). Such treatments could be film
forming (paint, stain, or clear coatings) or penetrating (water
repellent).

However, many bridges do not have such finish treatments. Figure
81 shows a covered bridge with aged, untreated siding. The bridge
was built in 1838 (making it one of the oldest in the United
States) and has had untreated siding since its original
construction.

Few, if any, historic covered bridges retain their original
siding. The original siding normally would have been replaced as
part of a general rehabilitation during the life of the bridge
(repairing bridge trusses normally requires removing at least some,
if not all, of the siding).

Specifying the subsequent siding replacement sometimes is
influenced by the protective treatment used on the previous siding.
If the siding was treated with a film-forming finish (paint or
stain), it may be duplicated to maintain the bridge's previous
appearance; if it was not treated with such a finish, the new
siding also may not be treated.

Some preservationists attach little importance to the color
appearance of the siding, because it is almost never original and
is considered a routine maintenance feature. Others believe that
color is very important for those bridges that have been painted
for a long time. In that case, one may analyze the paint on the
bridge to determine compositions and colors of the remaining paint;
this can guide the paint selection for the rehabilitated
structure.

Although an effective coating of paint or stain can prolong the
life of the siding, untreated board siding usually lasts for many
decades. By the time untreated siding requires replacement, the
trusses often require work as well, so a general rehabilitation
contract can address the siding at that time. In other words,
treating the siding has more of an aesthetic impact than any
special effect on the bridge's longevity.

The decision to surface-treat the siding includes the obligation
to maintain the treatment during the life of the siding. Because
most covered bridges are over water, renewing the surface treatment
of the siding is often a difficult and expensive operation. This
may be the most practical issue related to the decision of whether
to surface-treat siding, and it explains why many covered bridges
are allowed to weather to natural browns and grays.